Apparatus for changing a sensing device

ABSTRACT

A device for changing styli or probes in a coordinate measuring machine has a magazine (330) with several stations each holding a stylus or probe (320). A holder (310) on the quill of the coordinate measuring machine has an opening (313A), so that kinematic support elements (325) in the holder (310) can be moved underneath cooperating kinematic support elements (324) on the stylus (320). The holder (310) is then lifted up to engage the kinematic support elements, and the stylus can then be carried horizontally out of the magazine (30). During this operation, the kinematic support elements are biased into engagement initially by a leaf spring (335), and subsequently by a spring-loaded arm (314) in the holder (310), which bears against the top of a plate (322) of the stylus (320). When the holder (310) is engaged in the magazine (330), an extension plate (334) causes the arm (314) to swing out of the way. This arrangement enables the holder (310 ) to pick up the stylus (320) purely by translational movements of the holder (310), without the need for any separate motor or electromagnet to couple the stylus to the holder.

This invention relates to apparatus for changing a sensing device in apositioning machine, such as a coordinate measuring machine, machinetool or inspection robot. Such apparatus is known, and comprises:

a magazine having means for releasably supporting the sensing device ina stored position;

a holder which is capable of translational movement by the positioningmachine, relative to the magazine and to a workpiece, for removal of thesensing device from the magazine, performance of a sensing operation onthe workpiece, and return of the sensing device to the magazine;

means for releasably coupling the sensing device to the holder,comprising first support elements provided on the holder, second supportelements provided on the sensing device and engageable with the firstsupport elements, and releasable bias means for urging the first andsecond support elements into engagement with each other; and

means for actuating the coupling means between a coupled state (in whichthe bias means urge the first and second support elements intoengagement with each other) and an uncoupled state (in which the biasmeans are released and the holder is separated from the sensing devicewith the sensing device supported in the magazine) in order to coupleand uncouple the sensing device and the holder.

In the known apparatus, the actuating means has a separate operatingmeans, located either in the holder or in the magazine.

For example, in GB 2047133 (Renishaw), the coupling means is in the formof a bayonet joint, and the holder includes a motor for rotating theholder relative to the sensing device stored in the magazine, in orderto actuate the bayonet joint. In U.S. Pat. No. 4649623 (Zeiss), thecoupling means includes an electromagnet in the holder, which forms partof the bias means for urging the first and second support elements intoengagement. Both these known arrangements thus require an operatingmeans (a motor or electromagnet) in the holder. There is a difficulty inmaking the necessary electrical connections to such an operating meansin the holder, bearing in mind that the holder is movable. Furthermore,the operating means increases the mass of the holder, so that thepositioning machine is more subject to inertial deflections when theholder is accelerated Since such deflections reduce the accuracy ofmeasurement, the mass of the holder is one of the limiting factors onthe speed at which measurements can be made.

In EP 0142373 (Renishaw), on the other hand, such problems are overcomeby providing the operating means within the magazine itself The magazineincludes a screwdriver-type operating member, driven by a motor, whichengages and operates the coupling means when the holder picks up or putsdown a sensing device in the magazine However, this results in a morecomplicated and expensive magazine, and the need to ensure correctoperating engagement between the screwdriver and the coupling means.

In one aspect, the present invention is characterised in that theactuating means is operable by the relative translational movementbetween the holder and the magazine. Such relative translationalmovement is in any case an operational requirement in a positioningmachine, and therefore the preferred embodiments of the presentinvention do not require any separate motor or electromagnet in theholder or in the magazine.

Preferred embodiments of the invention will now be described withreference to the accompanying drawings, wherein: FIG. 1 is an elevationof a positioning machine including the connecting mechanisms FIG. 2 is aview on the line II--II in FIG. 1. FIG. 3 is an enlarged detail of FIG.2. FIG. 4 is a section on the line IV--IV in FIG. 3. FIG. 5 is a sectionon the line V--V in FIG. 3. FIG. 6 is a view similar to FIG. 4 but showsa first modification. FIG. 7 is a section on the line VII--VII in FIG. 6and shows different operational positions. FIG. 8 is view similar toFIG. 6 but shows a second modification and shows a first operationalposition. FIG. 9 is a section on the line IX--IX in FIG. 8. FIG. 10 is aview similar to FIG. 8 but shows a second operational position. FIG. 11is a section on the line XI--XI in FIG. 10. FIG. 12 is a view similar toFIG. 10 but shows a third operational position. FIG. 13 is a section onthe line XIII--XIII in FIG. 12. FIG. 14 is an elevation of a furtherpositioning machine. FIG. 15 is a section on the line XV--XV in FIG. 14.FIG. 16 is an enlarged detail of FIG. 14 and shows a first operationalposition FIG. 17 is a section on the line XVII--XVII in FIG.16. FIGS.18,19 and 20 show respectively a second, third and fourth operationalposition of the device of FIG.16. FIG. 21 is a view similar to FIG.16but shows a modification. FIG. 22 is a vertical section of anothermodification of the device of FIG. 16.

FIG. 23 is a section on the line XXIII--XXIII in FIG.22.

FIG. 1 shows a coordinate measuring machine having an arm member orquill 11 supported for translational motion only in the X,Y andZ-direction of the orthogonal coordinate system. In the example the armmember or quill 11 is a vertically disposed elongate member, supportedfor longitudinal motion in the Z-direction in a bearing 12 integral witha carriage 13 supported for motion in the X-direction along a beam 14itself supported for motion in the Y-direction on a track 15 mounted ona table 16 on which a workpiece 17 to be measured is supported. Themember 11, carriage 13 and beam 14 are adapted to be driventranslationally by motors (not shown) through any preselected linear orcurved path under the control of an appropriately programmed computer(not shown), this being well-understood per se.

Measurements are performed by means of a probe unit 10 having an axis10A and comprising a holding unit 20 and a stylus unit 19. The holdingunit is secured to the member 11. The stylus unit 19 is one of a numberof such units adapted for different measuring operations and held in amagazine 18 supported on the table 16. The computer is programmed todrive the member 11 to perform measuring operations and to transfer anyone of the units 19 between the holding unit 20 and the magazine 18.

Referring primarily to FIGS. 2 to 4, the unit 19 comprises a stylus 21and a first connecting member 22 The holding unit 20 comprises a secondconnecting member 23 engageable with the member 22. The member 22comprises a disc-shaped head 24 and three cylindrical arms 25 extendingradially outwardly from the head 24 and being equi-spaced at 120° (FIG.3). The arms 25 define open spaces 25B angularly therebetween. Themember 23 comprises housing 26 which is open at its bottom end exceptfor three inwardly extending flanges 29 Each flange has secured theretoa pair of spherical supports 30 defining between them a pair ofconverging surfaces 30A (FIG.3). The pairs of supports 30 are alsoequi-spaced at 120 degrees about the axis 10A. The flanges 29 arediscontinuous so as to define an open space 30B angularly between eachtwo pairs of supports 30 The supports 30 and arms 25 define first andsecond support elements respectively The upper end of the housing 26 hassecured thereto one end of a compression spring 31 (FIG. 4). The otherend of the spring 31 has secured thereto a pressure member 31Aterminating at a pointed end 31B.

FIGS. 1 and 4 show the units 19,20 in the disengaged condition but inalignment on the axis 10A. The unit 19 is supported (FIGS. 3 and 4) on asurface 33 of an arm 35 . of the magazine 18. When it is required forthe quill 11 to pick up the unit 19, the quill is lowered so that theelements 25,30 pass through the spaces 30B,25B (position 35B, FIG. 3)and the end 31B abuts the bottom of a conical recess 24A (FIG. 4) in thetop of the head 24 the top of the head 24. Further lowering of the quill11 compresses the spring 31 and brings the arms 25 into a position abovethe level of the spherical supports 30. The units 19,20 are thenrotated, one relative to the other, through 60° to bring the arms 25into axial alignment with the respective pairs of convergent surfaces30A, and the quill 11 is raised again whereby the spring 31 becomes freeto urge the head 24 downward and thereby resiliently urge the arms 25into engagement which the surfaces 30A. The arms 25 and convergentsurfaces 30A constitute a kinematic support means ensuring, incooperation with the spring 31, that the units 19,20 are located againstrelative displacement transversely to and rotationally about the axis10A, this condition being referred as the "rest position" of the stylus21.

For the purpose of rotating the units 19, the magazine 18 comprises aplurality of said arms 35 each supported on a frame 36 for pivotalmotion about a respective axis 35A extending in the Z-direction. Thearms are connected by a link 37 so as to be moveable in mutuallyparallel relationship, though this link can be omitted if the arms 35are sufficiently spaced apart. Detent means (not shown) are provided forlocating the arms 35 with a light force at the position 35B and afurther position 35C, (FIG.3). The positions 35B,35C define between theman angle W of 60°. Each arm 35 has a slot 32 in which the unit 19 issupportable against rotation relative to the arm so as to maintain therequired orientation of the unit 19 relative to the supports 30.

The pivotal motion of the arms 19 is effected by operation of themachine to cause translational movement of the quill through 60° of anarc 11A about the axis 35A of the relevant arm 35. This causes saidrelative rotation between the units 19,20 such that, on completion ofthe movement of the quill the elements 25,30 are in alignment in theZ-direction as shown at position 35C. The quill is then raised wherebythe elements 25,30 come into engagement and the unit 19 becomes seatedon the elements 30. The unit 19 can be returned to the magazine 18 bythe reverse of the operations described.

When any of the stylus units 19 is engaged with the support unit 20 toconstitute the probe assembly 10, the latter can then be used formeasuring the workpiece 17 by the process of moving the quill 11 toengage the stylus 21 with any selected surface of the workpiece. Theengagement generates a signal 39 (FIG. 3), and the position of saidsurface is determined from a reading of the position of the quillrelative to a pre-selected datum at the instant of the signal 39. Thisprocess of signal generation is known per se. The signal 39 may begenerated by connecting the supports 30 in an electric circuit 38capable of being closed by the arms 25, the signal being generated whenone or more of the arms 25 are displaced from the relevant supports 30when, on engagement with the workpiece, the stylus is displaced from itsrest position Alternatively, a piezo-electric or other force-sensitiveelement 40 may be provided on the pressure member 31A for generating thesignal 39 in response to the sound wave produced in the stylus onengagement with the workpiece.

The unit 19 includes a cup-shaped seal element 42 having an annular wall42A forming a seal 41 (FIG. 5) with an annular groove 43 of the unit 20thereby to protect the elements 25,30 against ingress of dust into thehousing 26.

Referring to FIGS. 6 and 7, there is shown a mechanism similar to thatshown in FIGS. 1 to 5 but modified to provide protection of the stylusunit while in position in the magazine. Like parts are given likereference numerals preceded by the numeral 1 Thus there is shown astylus unit 119 having arms 125 protected by an annular wall 142, and aholding unit 120 having supports 130 inside a housing 126 which, in thiscase, has a cylindrical exterior. The magazine, here denoted 118, has inrespect of each unit 119 a lower arm 135 for the support of the unit119. The modification is as follows.

The arm 135 is supported for pivotal motion about an axis 135A by meansof a shaft 150 arranged in a bearing 151 provided on the magazine 118.The arrangement is such that in the stored position of the unit 119 inthe magazine, the arm 135 has a position 135D (FIG. 7) such that theunit 119 is situated wholly beneath a horizontally diposed panel 152 ofthe magazine, and the arms 125 of the unit 119 are protected by thepanel 152. In particular the upper edges of the wall 142 form a seal 153with the underside of the panel 152 against the ingress of dust.

The unit 119 is supported on a surface 133 of the arm 135 while acylindrical portion 134 of the unit 119 is in engagement with anopen-ended slot 132 of the arm 135. A short pin 154 engages with a hole155 to prevent rotation of the unit 119 relative to the arm 135.

Above the panel 152 the shaft 150 has secured thereto an upper arm 156which is generally parallel to the lower arm 135. The arm 156 comprisesan open-ended slot 157 dimensioned to receive the outside of the housing126 in a position in which the units 119,120 are in alignment on theaxis 10A.

For the purpose of engaging the unit 120 with the unit 119 and removingthe latter from the magazine, the computer is programmed to lower themember 11 translationally along the axis 10A into engagement with theslot 157 and thereafter move the member 11 translationally through anarc 111A sufficient for to bring the unit 119, which necessarilyaccompanies the movement of the arm 156, from the position 135D into aposition 135B in which the unit 119 is clear of the panel 152. Themember 11 is then further lowered. The housing 126 slides through theslot 156 into a position in which the supports 130 are below the levelof the arms 125. Thereafter the movement of the member 11 through thearc 111A is continued by a further 60° with the result that the arms 125attain a position of vertical alignment with the support 130. Thereafterthe member 11 is raised to engage the arms 125 and supports 130, andfurther raising movement of the member 11 disengages the pin 154 fromthe hole 155. The unit 119 is now free to be removed from the arm 135either by further raising of the member 11 or by movement of the member11 in a direction out of the slots 132,156.

Instead of being moved through the arc 111A, the member 11 may be movedalong chords 111B,111C to effect the pivotal motion of the arm 135 and,in the case of the chord 111C, to effect the relative rotation of theunits 119,120. However, such a manoeuvre requires the unit 120 to sliderelative to the arm 156, and the slot 157 must be made sufficiently longto accommodate such sliding. Similarly, during movement of the member 11along the chord 111C, the unit 119 has to slide along the slot 132 andthe latter has to be sufficiently long to accommodate the slidingmovement of the portion 132, the hole 155 being dispensed with and thepin 154 being adapted to slide along the appropriately extended slot132.

It will be seen that the member 11 is used to perform two functions,viz. firstly, moving the arm 135 between the positions 135D,135B for thepurpose of moving the unit 119 out of and into the protected positionbelow the panel 152 and, secondly, moving the arm 135 between thepositions 135B,135C for the purpose of relatively rotating the units119,120 for effecting their engagement or disengagement. These twofunctions may be carried out separately. For example, the first functioni.e. the movement between the positions 135D,135B, may be carried out bya solenoid or other motor device because here no relative rotationbetween the units 119,120 is of significance.

Generally, in both examples described, the operation of engaging ordisengaging the units 19,119;20,120 is effected solely by translationalmovement of the member 11, i.e. by means of the motors already providedfor moving the member 11 for measuring purposes, so that no additionalelectrical devices are required for effecting said engagement ordisengagement.

It will be understood that instead of using the magazine 118 for storingstylus units 119, the magazine may store one or more probe units, i.e.units 110 each embodying a pair of units 119,120 in a state ofengagement, and where each unit 110 has further arms similar to the arms125 for engagement with the supports 130 by manoeuvres similar to themovement of the member 11 between the positions 135D,135B,135C or135B,135C.

Referring to FIGS. 8 to 13, there is shown a mechanism similar to thatshown in FIGS. 6 and 7 but showing a further or second modification.Like parts are given like reference numerals preceded by the numeral 2.There is shown a stylus unit 219 having an axis 19A and a holding unit220 having an axis 20A. The stylus unit 219 includes a stylus 221 andthree extensions or arms 225 extending radially from the axis 19A. Thearms 225 are formed by the corner portions of a triangular plate 224,and each has a spherical support element 225A on its underside. Thecorner portions act to cover and protect the elements 225A from dirt anddamage The unit 219 also includes a cup-shaped seal element 242.

The holding unit 220 includes a circular plate 229, and a housing 226whose exterior is cylindrical at least at the lower end portion thereofThree symmetrically spaced supports 230 are provided on the plate 229 inthe form of three V-grooves extending radially from the axis 20A andengageable kinematically by the respective elements 225A. The plate 229has, at its centre, a triangular cut-out 229A the corner portions ofwhich define open spaces between each pair of V-grooves 230 so that thearms 225 can pass through this cut-out in operation The upper end of thehousing 226 has secured thereto one end of a compression spring 231. Theother end of the spring 231 has secured thereto a pressure member 231Aterminating at a pointed end 231B for engagement with the plate 224 tourge the elements 225A resiliently into engagement with the grooves 230.

The magazine, here denoted 218 has, in respect of each unit 219 to bestored therein, a lower arm 235 on which the unit 219 is supportable.Each arm 235 is supported for pivotal motion about an axis 235A of ashaft 250 arranged in a bearing 251 provided on the magazine 218. Thearrangement is such that in the stored position of the unit 219 in themagazine, the arm 235 has a position such that the unit 219 is situatedwholly beneath a horizontally disposed panel 252 of the magazine (FIGS.8,9), and the arms 225 of the unit 219 are protected by the panel 252.The free edges of the cup-shaped seal element 242 form a seal 253 withthe underside of the panel 252 against the ingress of dust.

The unit 219 is supported on a surface 233 of the arm 235 while a neck234 of the unit 219 is in engagement with an open-ended slot 232 of thearm 235. The neck 234 has parallel sides 254 which engage the slot 232in a manner preventing rotation of the unit 219 relative to the arm 235.

Above the panel 252 each shaft 250 has secured thereto an upper arm 256which is generally parallel to the lower arm 235 (as seen in FIG. 8) andwhich comprises an open-ended slot 257 dimensioned to receive theoutside of the housing 226 in a manner permitting rotation of the arm256 relative to the housing 226. It is clear that the arms 235,256 aremovable jointly about the axis 235A. Relevant angular positions of thearm 235 during such movement and the corresponding positions of the unit219, are denoted 19A1,19A2,19A3. The corresponding positions of the arm256 and of the unit 220 are denoted 20A1,20A2,20A3.

For the purpose of engaging the unit 220 with the unit 219, the computeris programmed to lower the member 11 along the axis 20A into engagementwith the slot 257 at the position 20A3, as seen in FIGS. 8 and 9.Thereafter, it translates the unit 220 linearly, firstly between thepositions 20A3 and 20A2 (FIGS. 10,11) and then between the positions20A2 and 20A1 (FIGS. 12,13). During those translations the housing 226co-operates with the slot 257 to rotate the arm 256 and thereby rotatethe arm 235 to move the unit 219 supported thereon through an arc 211Abetween the positions 19A3,19A2 and 19A1 respectively. The directions ofthe translations between the positions 20A3,20A2 and 20A2,20A1 arealigned to the x and y directions of the translational motions of themachine (FIG. 1) so that the movement of the arm 235 can be effectedconveniently by simple operation of the machine. In the positions19A3,20A3 it does not matter whether the axes 19A,20A are in aligmentbut the arrangment is such that in the positions 19A2,20A2 and 19A1,20A1the axes 19A,20A are in alignment.

During the translation of the unit 220 from the position 20A3 to theposition 20A2 the unit 19 is moved from under the panel 252 into aposition confronting the unit 220 (FIGS. 10,11) The member 11 is thenfurther lowered so that the housing 226 slides through the slot 257 intoa position in which the supports 230 are below the level of the arms225. Thereafter linear movement of the member 11 to translate the unit220 from the position 20A2 to the position 20A1 causes the arm 235 torotate through 45° relative to the unit 220 with the result that thearms 225 attain a position of vertical alignment with the supports 230(FIGS. 12,13). Thereafter the member 11 is raised to engage the arms 225and supports 230. The unit 219 is now free to be removed from the arm235 by movement of the member 11 in a direction out of the slots232,256. The seal member 242 is received with a substantial gap in anannular groove 243 in the housing 226, in order form a seal while stillpermitting the stylus unit 219 to tilt.

Further details of the present modification will now be described.

The stylus 221 is displaceable relative to the plate 229 in directions±X,±Y and +Z, such displacement being accommodated by tilting of theplate 224 on the plate 229 or by axial translation of the plate 224towards the housing 226, all these displacements being accommodated bycompression of the spring 231. In the present modification the stylus isalso displaceable in the -Z direction. This is achieved by the virtue ofthe plate 229 being supported on the housing 226 by three sphericalelements 226A secured to the housing 226 and engaged respectively bythree further V-grooves 229B in the plate 229. The grooves 229B aresymmetrically spaced with respect to each other about the axis 20A, andangularly spaced from the grooves 230 The plate 229 is urged resilientlyinto the position of engagement with the elements 226A by three tensionsprings 258 (only one visible in FIG. 10) connected between the plate229 and the housing 226 The -Z displacement is accommodated by extensionof the springs 258 When the displacing forces cease the spring 231 orthe springs 258, as the case may be, return the stylus unit to itsoriginal or rest position.

The pressure member 231A has an upwardly extending spigot 280, the topof which is displaceably steadied by a diaphragm 281. The spigotcontains a lens 284 for focussing light from a light emitting diode 283onto a photodetector 282. When the stylus 221 contacts a workpiece, thelens is disturbed slightly, and the resulting change in the output ofthe photodetector is easily detected

The magazine 218 has vertical panels 259,260 integral with thehorizontal panel 252 to further enclose and protect the stylus units219. The panel 259 has an opening 261 for access of the stylus units 219to the stored position 19A3 and each arm 235 of the magazine 218 hassecured thereto a door 262 which closes the opening when the respectivestylus unit is in the stored position.

Referring to FIG. 14 and 15, a coordinate measuring machine CMMcomprises an elongate operating member or quill 311 supported on a base331 for three-dimensional orthogonal movement in directions X,Y,Z. Amagazine 330 provided on the base 331 is adapted to support a pluralityof styli 320 at respective stations 332 of the magazine. A stylus holder31 0 provided on the operating member 311 is adapted to be moved betweenany position A in the measuring field of the machine and a position B(FIG.16) adjacent to any one of the stations 332.

At each station 332 (FIG.16,17), the magazine 330 comprises a support333 for a said stylus 320. The stylus 320 comprises an elongate member321 secured at one end to a plate 322. The plate 322 is provided at theside adjacent the member 321 with three elements 324 of a kinematicsupport 323 the other elements 325 of which are provided on the stylusholder 310. The support 333 is fork-shaped in the sense of having twoprojections 333A for supporting the plate 322 at opposite sides of themember 321 and so that the stylus 320 can be withdrawn from the support333 in a direction transverse to the length of the member 321. A lightleaf spring 335 holds the stylus 320 gently in place on the support 333,located by the forks 333A and by a peg 380 in a hole 381 in the support333.

The holder 310 has a housing 312 including a fork-shaped support 313 onwhich said other elements 325 are provided. The gap between these forksis wide enough to straddle the support 333. Further, the holder 310 hasa pressure device 314 being in the form of an arm 315 connected at oneend to the housing 312 by a pivot 316 and generally being situated in aposition confronting the elements 325. The other end of the arm isprovided with a roller 317. The arm 315 is urged against a stop 318 by atorsion spring (not shown) at the pivot 316 and is withdrawable from thestop 318 generally in the direction away from the open end 313A of thesupport 313.

In operation, when it is required for the holder 310 to pick up a stylus320 from the magazine 330, the operating member 311 is moved into theposition B adjacent the relevant magazine station 332, as shown inFIGS.16 and 17. In that position the open ends of the fork-shapedsupports 313,333 confront each other in the X direction, the support 333being situated at a level below the support 313 and the arm 315 beingsituated at a level generally above the plate 322.

Thereafter the member 311 is moved in the X-direction toward the stylus320 until the holder 310 attains a position C (FIG.18) in which theelements 324,325 confront each other in the Z-direction. During themovement between the positions B,C there occurs engagement between theroller 317 and a release member or extension 334 provided on themagazine 330 in a position above the plate. The arrangement is such thatthe extension 334 raises the arm 315 and prevents engagement thereofwith the plate 322.

Following attainment of position C, the member 311 is moved to move theholder 310 upwards in the Z-direction into a position D (FIG.19) inwhich the elements 324,325 become engaged and, following suchengagement, the plate 322 is lifted clear of the support 333.

Lastly, the member 311 is moved in the X-direction (FIG.20) in the senseof retracting the holder 310 from the magazine 330 The engagement of theelements 324,325 is now substantially sufficient for the stylus 320 tobe removed from the magazine together with the holder 310 but the lightleaf spring 335 under the extension 334 assists in holding the elements324,325 in engagement during the movement. Toward the end of themovement the arm 315 drops away from the extension 334 and the roller317 drops onto the top of the plate 322 of the stylus 320 as supportedon the elements 324,325.

The length of the arm 315 is so chosen that a relatively large angle Wis formed between the arm and the plate 322 and a correspondingly largedownward pressure is exerted on the plate 322 to retain the stylus inposition with appropriate firmness.

When it is desired to return the stylus to the magazine, the holder issimply returned to position D, lowered into position C and withdrawninto position B The extension 334 removes the arm 315 from engagementwith the plate 322 as the position D is approached.

In a modification (FIG.21) a pressure member 326 is provided on the topof the plate 322 and spring-loaded by a spring 327 so that when thestylus is engaged with the magazine, member 326 engages a roof portion340 of the magazine and urges the plate 322 against the support 333. Theholder 310 is provided with a forked extension 319 adapted to straddlethe member 326 and engage ramps 341 on each side of the member 326 tourge the latter toward the plate 322 against the pressure of the spring327. Subsequent lifting of the holder 310 engages the elements 324,325under the pressure of the spring 327, and the stylus is removable by theholder from the magazine The revese operation enables the stylus to bereturned to the magazine. When held in the holder, the pressure member326 acts against the forks 319 to urge the kinematic support elementsinto engagement with each other.

Another type of device which can be stored in one or more of thestations of the magazine is an extension bar, used to lengthen thestylus or probe in order to inspect relatively inaccesible parts of aworkpiece. Such extension bars are known, and in the present case wouldhave a plate such as 322 at the top end, with a holder similar to theholder 310 at the bottom end. Once such an extension bar has been pickedup from one station of the magazine, therefore, it can then go on topick up a stylus or probe from another station of the magazine.

FIGS. 22 and 23 show another modification of the device of FIG.16. Aspreviously, a magazine 430 has stations for holding several styli 420,which can be picked up from and deposited at their respective stationsby a holder 410 on the quill of the measuring machine. At each station,the magazine 430 has a forked support 433, which supports the stylus 420between its forks 433A. The elongate member 421 of the stylus 420 has aflat portion 450 machined on each of its sides, forming grooves whichengage with the forks 433A. The forks have a circular recess 451 intheir upper surface. When the stylus 420 is held by the support 433,therefore, the forks 433A cooperate with the grooves 450 to ensure thatthe stylus is correctly oriented about a vertical axis for presentationto the holder 410, while the circular recess 451 locates the cylindricalsurface of the elongate member 421 in the correct location in thehorizontal plane.

As previously, the stylus 420 has a plate 422, on the underside of whichare balls 424 which engage with pairs of rollers 425 on the holder 410,to provide a kinematic support for the stylus in use. The balls 424 arebiased into engagement with the rollers 425 in use by a roller 417 on anarm 415, under the loading provided by a torsion spring 452.

One important modification of the present device over that of FIG.16 isthe provision of a manually-operable button 460, which enables the arm415 to be rotated manually about its pivot point 416, out of contactwith the plate 422 of the stylus This enables the stylus to be removedfrom the holder 410 by hand if desired The button 460 has two slots 461which cooperate with two screws 462 to enable the button 460 to slidehorizontally in and out. It is biased into the radially outward positionby a compression spring 463. The button 460 has a nose 464 which engagesthe arm 415 when the button is depressed against the action of thespring 463.

It would be possible to provide a fixed extension plate above thesupport 433, which would engage the button 460 during the normalautomatic stylus changing operation, in exactly the same way as theextension plate 334 in the device of FIG.16. Such an extension platewould of course need to be a little shorter than the extension plate334, to take account of the depth of the button 460.

However, in the device as shown in FIG.22, we instead provide a push rod465 at each station of the magazine 430. The rod 465 is positioned toengage the button 460 as the holder 410 approaches the forks 433A, andto slide backwards in a bore 466 when it does so This backwards slidingis against the action of a spring 467. The spring rates and pre-loadingsof the springs 463, 467 are chosen, relative to each other, such thatthe pressure of the arm 415 on the plate 422 is applied and releasedmore gradually than would be the case with a fixed extension such as334. Thus, when the holder 410 is depositing the stylus 420 in themagazine staion, and brings the stylus horizontally so that the grooves450 engage in the forks 433A, it is possible to arrange that there willstill be a light pressure from the arm 415 holding the stylus gently inposition in the holder. This replaces the light pressure applied by theleaf spring 335 in the corresponding position shown in FIG.19. The lightpressure ensures that the stylus is held in position until the holder410 is lowered to disengage the balls 424 and rollers 425, whereupon thestylus is lowered into the recess 451 which then provides the necessarylocation to hold the stylus in position. Similarly, because the rod 465can also be arranged to allow the arm 415 to engage the plate 422 with alight pressure during the pick-up operation, this ensures that the balls424 and rollers 425 are sufficiently biased together to enable theholder 410 to remove the stylus 420 horizontally from the magazinestation once it has picked it up. As the stylus is removed horizontallyby the holder 410, of course, the rod 465 disengages from the button 460to ensure that the arm 415 holds the stylus 420 securely in itskinematic support.

FIGS.22 and 23 also show an arcuate cover 480 which is attached to theplate 422 of each stylus in the magazine. This arcuate cover 480 closesthe opening 413A of the holder 410 when the stylus is in position in theholder, so as to protect against the ingress of dust and othercontaminants.

All the embodiments shown in the accompanying drawings have had stylusunits stored in the stations of the magazine, and the means fordetecting when the stylus contacts a work piece has been in the holdermounted on the quill of the machine. However, if desired, a completeprobe unit may be stored in each of the stations of the magazine, in anyof the embodiments described Taking the embodiment of FIGS.16 and 17 asan example, this simply entails replacing the elongate stylus member 321with a complete probe of known design such as shown in U.S. Pat. Nos.4,153,998 or 4,702,013. This involves the addition of the plate 322 withits support elements 324 to the top of the existing design of probe.

The electrical connections for the probe can be in the form of contacts370 on the upper surface of the plate 22, which mate with correspondingcontacts 371 on the housing 312 when the kinematic support elements 324,325 are engaged. One or other of the sets of contacts should be lightlysprung to ensure proper electrical contact without interfering with thekinematic support. Where such electrical connections are provided, it isparticularly advantageous that they should confront each other in thesame direction as the direction of movement which brings the kinematicsupport elements together during the automatic change operation. Thisensures that there is no risk of the contacts fouling and damaging eachother, as might happen if they were bought together in a transversedirection. Of course, the electrical contacts 370, 371 are omitted inthe case of a simple stylus 320 not requiring any electricalconnections.

It is among the advantages of the various embodiments described abovethat the sensing device, (eg. a stylus, extension bar or probe) can betransferred between the holder and the magazine without the need for anyelectrical device eg. a motor or a magnet in either the magazine or theholder. In each case, the sensing device is picked up and depositedsimply by the translational movements of the holder, in the X,Y and Zdirections, which are the normal movements that the positioning machinewould be designed to make in any case.

Another important advantage of the embodiments described is that thestylus, probe or extension bar is supported kinematically when it is inthe holder Thus, if it is redeposited in the magazine, it cansubsequently be picked up again, and the kinematic seating will ensurethat it occupies exactly the same position with respect to the holder aspreviously. There is therefore no need for the stylus or probe to beredatumed or recalibrated each time it is picked up.

Although described with respect to coordinate measuring machines, thechanging apparatus described can of course equally be used to changesensing devices or other tools, eg. cutting tools, in a machine tool. Itcan also be used to change sensing devices in an inspection robot.

We claim:
 1. Apparatus for changing a sensing device in a positioningmachine, comprising:a magazine having movable support means forreleasably supporting the sensing device in a stored position; a holderwhich is capable of translational movement by the positioning machine,relative to the magazine and to a workpiece, for removal of the sensingdevice from the magazine, performance of a sensing operation on theworkpiece, and return of the sensing device to the magazine; means forreleasably coupling the sensing device to the holder, comprising firstsupport elements provided on the holder, second support elementsprovided on the sensing device and engageable with the first supportelements, and releasable bias means for urging the first and secondsupport elements into engagement with each other; and means foractuating the coupling means between a coupled state, in which the biasmeans urge the first and second support elements into engagement witheach other, and an uncoupled state, in which the bias means are releasedand the holder is separated from the sensing device with the sensingdevice supported in the magazine, in order to couple and uncouple thesensing device and the holder; characterized in that the actuating meansis operable by relative rotation between the holder and the sensingdevice, the relative rotation being caused by translational movement ofthe holder moving said movable support means.
 2. Apparatus according toclaim 1, wherein the first and second support elements form a kinematicsupport.
 3. Apparatus according to claim 1, wherein the support elementsare arranged to confront each other in a first direction, and areengaged by moving the holder to a first position in which the firstsupport elements lie beyond the second support elements in said firstdirection, and then moving the holder back along said first directioninto a second position in which the first support elements engage withthe second support elements.
 4. Apparatus according to claim 3, whereinthe holder is moved into the first position along a second directiontransverse to the first direction.
 5. Apparatus according to claim 3,wherein the sensing device is removed from the magazine by movement ofthe holder from the second position in a second direction transverse tothe first direction.
 6. Apparatus according to claim 4, wherein theholder is moved into the first position by forward movement along thefirst direction, followed by said translational movement of the holderin said second direction.
 7. Apparatus according to claim 6, wherein themovable support means comprises a pivotal support for supporting thesensing device, the pivotal support being operatively engageable withthe holder so as to be pivotal by said translational movement of theholder, thereby causing said relative rotation to provide angularalignment of said first and second support elements.
 8. Apparatusaccording to claim 7, wherein the pivotal support is movable between aposition in which the sensing device is housed under a panel of themagazine, and a position in which it is accessible to the holder; andwherein the pivotal support has an operating member outside said panelwhich is engageable by the holder in order to move the support from thehoused position to the accessible position.
 9. Apparatus according toclaim 1, wherein the bias means is on the holder, and the magazine hasan element for engaging the bias means, said element being brought intoengagement with the bias means by the translational movement of theholder.
 10. Apparatus according to claim 7, wherein the pivotal supportis engageable with said holder through a rotatable coupling between saidholder and said sensing device.
 11. Apparatus according to claim 7,wherein there are provided on the magazine a plurality of pivotalsupports connected by a link so as to be moveable in a mutually parallelrelationship.
 12. A method of releasably coupling a sensing deviceretained in a magazine on a pivotal support, to a holder supported by apositioning machine for movement relative to the magazine, wherein thesensing device and holder are coupled by relative rotation, the methodcomprising the steps of:moving the holder to a predetermined positionrelative to the sensing device; with the sensing device retained in themagazine, translating the holder and sensing device together, thereby tocause the support to pivot, the sensing device to rotate relative to theholder and to effect coupling between the sensing device and holder; andwith the sensing device coupled to the holder, translating the holder toremove the sensing device form the magazine.
 13. A method according toclaim 12, wherein the holder is brought into said predetermined positionby translating the holder in a first direction, and the holder andsensing device are rotated relative to each other by translating theholder and sensing device in a second direction transverse to the firstdirection.
 14. A method according to claim 13, wherein a set of firstsupport elements are provide don the older, and a set of second supportelements engageable with the first support elements are provided on thesensing device, and wherein when the holder is in the predeterminedposition ,the first and second support elements are angularly offsetfrom each other about an axis parallel to the first direction, and arespaced apart in the first direction.
 15. A method according to claim 14,wherein when the holder and sensing device have been translated in thesecond direction, the holder is moved in a direction opposite to saidfirst direction to being the first and second support elements intomutual engagement.
 16. A coordinate positioning machine having:amagazine for retaining at least one sensing device upon a pivotalsupport; a holder for the sensing device, the holder being supported bythe machine on a movable arm for translational movement relative to themagazine; and coupling means actuable by relative rotation of the holderand sensing device for releasably coupling the at least one sensingdevice to the holder, said relative rotation being caused bytranslational movement of the holder pivoting said pivotal support. 17.A machine according to claim 16, further comprising:a set of firstsupport elements provided upon the holder and a set of second supportelements engageable with the first support elements provided upon saidat least one sensing device; an aperture provided in the holder forreceiving the second support elements provided on said at least onesensing device; wherein the second support elements can only passthrough the aperture when the first and second support elements areangularly offset from each other about an axis parallel to the directionof passage of the second support elements through the aperture.
 18. Amachine according to claim 17, wherein the first and second supportelements provide a kinematic support for the at least one sensing deviceon the holder.
 19. A machine according to claim 17, wherein the at leastone sensing device has a cup-shaped seal, for cooperating with theholder to prevent ingress of dust in the holder when the holder andsensing device are coupled.
 20. A machine according to claim 19, whereinsaid first and second support elements, when engaged, are incorporatedinto an electrical circuit.